Radio-frequency filters based on BAW resonators are of great interest for many RF applications. Substantially, there are two concepts for BAW resonators, so-called thin film BAW resonators (FBAR), on the one hand, as well as so-called solidly mounted resonators (SMR). Thin film BAW resonators include a membrane on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. The acoustic resonator develops by the reflection at the upper side and at the lower side of the membrane. In the alternative concept of solidly mounted resonators, an SMR includes a substrate, for example a silicon substrate, on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. So as to keep the acoustic waves in the active region in this design, a so-called acoustic mirror is required. It is located between the active layers, i.e. the two electrodes and the piezoelectric layer, and the substrate. The acoustic mirror consists of an alternating sequence of layers with high and low acoustic impedance, respectively, e.g. layers of tungsten (high acoustic impedance) and layers of oxide material (low acoustic impedance). In the following, layers of high or low acoustic impedances, respectively, are understood to mean layers which define, when superimposed, a transition area where acoustic waves are reflected; to be precise, the larger the difference between the acoustic impedances of the layers, the higher the intensity with which acoustic waves are reflected.
If the mirror contains layers of conducting materials, such as tungsten, it is recommended, for the avoidance of parasitic capacitances in the filter, to structure (pattern) and substantially limit the corresponding mirror layers to the area below the active resonator region. The disadvantage of this procedure is that the topology resulting hereby cannot be completely planarized. Due to the unevenness, undesired modes are induced in the resonator and/or a reduction in the quality of the resonator is caused. This problem is very critical in so far as already small steps or remaining topologies of several percent of the layer thickness have significant influence on the operation behavior of such a resonator.
On the basis of FIGS. 4 and 5, two known methods of manufacturing acoustic mirrors for piezoelectric resonators or BAW resonators are explained in greater detail.
FIG. 4 shows a solidly mounted resonator with structured mirror. The resonator includes a substrate 100 with a lower surface 302 and an upper surface 304. A layer sequence 306 forming the acoustic mirror is arranged on the upper surface. Between the substrate and the mirror, one or more intermediate layers serving for stress reduction or adhesion improvement may be arranged, for example. The layer sequence includes alternately arranged layers 306a with high acoustic impedance and layers 306b with low acoustic impedance, wherein intermediate layers may be provided between the mirror layers. On the upper surface 304 of the substrate 100, a first layer 306b1 with low acoustic impedance is formed. On the layer 306b1, a material 306a1, 306a2 with high acoustic impedance is deposited and structured at the portions associated with the active regions of the resonator. Over this arrangement, a second layer 306b2 with low acoustic impedance is deposited, upon which in turn a material 306a3, 306a4 with high acoustic impedance is deposited and structured section-wise. Upon this layer sequence, again a layer with low acoustic impedance 306b3 is deposited. On the resulting mirror structure, a lower electrode 310, on which again the active or piezoelectric layer 312, for example of AlN (AlN=aluminum nitride), is arranged, is at least partially formed. On the piezoelectric layer 312, an insulation layer 314 covering the piezoelectric layer 312 except for the regions 316a1 and 316b1 is formed. Two upper electrodes 3181 and 3182 in contact with the piezoelectric layer in the portions 3161 and 3162 are formed on the piezoelectric layer. A tuning layer 3201 and 3202, via the thickness of which a resonance frequency of the resonators can be adjusted, is at least partially arranged on the upper electrode 3181, 3182. By the portions of the upper electrode 3181 and 3182 in which it is in connection with the piezoelectric layer 312, and the underlying portions of the lower electrode 310, two BAW resonators 322a and 322b are defined. The mirror structure 306 shown in FIG. 4 includes λ/4 mirror layers 306a, 306b. 
In the example of a solidly mounted resonator shown in FIG. 4, the metallic layers 306a are structured without planarizing the resulting topology. The layers 306b with low acoustic impedance are deposited over the structured layers 306a, as described above. Thereby, the steps shown in FIG. 4, which continue in the deposition of the overlaying layers, develop. This procedure is disadvantageous regarding the resulting strong topology in the layers lying above the mirror 306, in particular, with reduced piezoelectric coupling of the active layer 312, low resonator quality or increased excitation of undesired vibrational modes arising, it being possible for this to lead to a total failure of the device.
FIG. 5 shows a further example known in the prior art for solidly mounted resonators with a structured mirror. In FIG. 5, again a substrate 100 is shown, on the upper surface 304 of which an oxide layer 324 is deposited, into which a pit or depression 326 is introduced. Further intermediate layers may be provided between the oxide layer 324 and the substrate 300. In the pit 326, the acoustic mirror is formed, which consists of a layer sequence comprising a first layer 306a1 with high acoustic impedance, a layer 306b with low acoustic impedance, and a layer 306a2 with high acoustic impedance. On the surface of the resulting structure, an insulation layer 308 is deposited, on which the lower electrode 310 is at least partially formed. The portion of the insulation layer 308 not covered by the lower electrode 310 is covered by a further insulation layer 328. On the insulation layer 328 and on the lower electrode 310, the piezoelectric layer 312 is formed, on the surface of which the upper electrode 318 is in turn partially formed. The portions of the piezoelectric layer 312 not covered by the upper electrode 318, as well as parts of the upper electrode 318 are covered by the passivation layer 314. The overlapping areas of lower electrode 310, piezoelectric layer 312, and upper electrode 318 define the BAW resonator 322.
In the example shown in FIG. 5, the pit 326, in which the mirror layers 306a, 306b are deposited one after the other, as described above, is etched into the oxide layer 324 in the area of the resonator 322 to be produced. By one or more CMP (chemical mechanical polishing) processes, the layers outside the mirror pit 326 are removed.
Both methods exhibit disadvantages. The method described by means of FIG. 4 exhibits, for example, the disadvantage of a high level of topology in the layers overlaying the mirror, which results in unfavorable conditions for further processing. This shows, among other things, in reduced piezoelectric couplings, low resonator quality or undesired modes up to a total failure of the device. The method described using FIG. 5 is disadvantageous in that in the corners of mirror pit 326, the layers are slightly thinner, and in that during planarizing, a slight dish topology, which is indicated by reference numeral 330 in FIG. 5, is formed in the resonator area 322, which in turn leads to increased excitation of undesired modes and to reduced resonator quality. Furthermore, the method described using FIG. 5 is disadvantageous in that the topmost mirror layer is attacked to varying degrees across the active area of the resonator in the various CMP steps, which leads to increased excitation of undesired modes and to reduced resonator quality. On the other hand, a certain residual topology cannot be avoided, which will lead to the above-listed disadvantages in subsequent processing. It is to be noted here that the residual topology created by the so called mirror pit and the subsequent CMP steps is a significant yield-limiting factor.